Self-made billionaire T. Denny Sanford, who made his fortune in subprime lending and credit card, is on track to making good on his goal to "die broke": he is giving $400 million to a South Dakota hospital to help find cures for children's diseases.

Sitting in Mr. Sanford's mountainside home in Vail, Colo., Mr. Krabbenhoft told the little-known billionaire: "Now I want to talk to you about something else."

He laid out plans for how his regional health-care system could make a national mark on the evolution of medicine, particularly in pediatrics. He compared his vision to the Manhattan Project, and talked about how Sioux Falls's geographic isolation might aid the heath-care system's efforts to make a world-changing medical discovery in the same way that the remoteness of Los Alamos, N.M., enabled the scientists who created the first atomic bomb to move forward quickly.

"How much is this going to cost?" Mr. Sanford asked.

"$400-million."

Mr. Krabbenhoft recalls Mr. Sanford clutching his hands to this throat — as if he were being choked. A long pause followed. The two then dug into the details of how the money would be spent. Five minutes later, Mr. Sanford said: "Let's get it done."